Hybrid powertrains include energy storage devices such as batteries which are utilized to reduce fuel consumption by capturing braking energy and by permitting more efficient use of an internal combustion engine. The engine may be shut off while the vehicle is stationary. Also, the engine may be operated at higher power setting at which it is typically more efficient and then shut off a portion of the time that the vehicle is moving.
One type of hybrid powertrain is an electric power-split hybrid. At low speed, a planetary gear set divides the mechanical power generated by the internal combustion engine into two power flow paths. A portion of the power is conveyed to the drive wheels by gears, chains, or other mechanical power transfer components. The remaining power is directed to an electric machine and converted into electrical power. This electric machine is typically referred to as a generator although it may also be capable converting electrical power into mechanical power. A second electric machine drives the drive wheels. This second machine is typically referred to as a traction motor although it may be capable of converting mechanical power into electrical power. In some operating modes, all electrical power from the generator flows to the traction motor. In other operating modes, some electrical power may be diverted to a battery. In yet other operating modes, the battery may supplement the electrical power.
Internal combustion engines typically are only capable of rotating in one direction and must rotate at a minimum speed in order to generate substantial propulsive power. Electric machines, on the other hand, can typically rotate in either direction and can develop torque in either direction even at very low rotational speeds. In a conventional power-split hybrid, the mechanical power flow path is not capable of propelling the vehicle in reverse. In reverse, the electric machines provide the bulk of the propulsion. Consequently, the torque capability is less in reverse than in forward. This performance reduction is particularly severe if the battery state of charge is low and engine power must be used.